$A$ nucleus ${}_{n}^{m}X$ emits one $\alpha$ particle and two ${\beta^-}$ particles. The resulting nucleus is

$X-$ It is very important for the process of evolution.
$Y-$ Crossing over is the exchange of genetic material between two homologous chromosomes.

Below are two statements. One is labeled as Assertion $(A)$ and the other as Reason $(R).$
Assertion $(A):$ Gypsum is used to make boards for fireproof walls.
Reason $(R):$ Gypsum is unstable at high temperatures.
In the context of the above statements,choose the correct answer from the options given below:

The major product of the following reaction is

Which one of the following pairs of compounds illustrates the law of multiple proportions?

$E_1: a+b+c=0$,if $1$ is a root of $ax^2+bx+c=0$. $E_2: b^2-a^2=2ac$,if $\sin \theta, \cos \theta$ are the roots of $ax^2+bx+c=0$. Which of the following is true?